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<a name="define_005fsubst-Example"></a>
<h4 class="subsection">17.21.1 <code>define_subst</code> Example</h4>
<a name="index-define_005fsubst-1"></a>

<p>To illustrate how <code>define_subst</code> works, let us examine a simple
template transformation.
</p>
<p>Suppose there are two kinds of instructions: one that touches flags and
the other that does not.  The instructions of the second type could be
generated with the following <code>define_subst</code>:
</p>
<div class="smallexample">
<pre class="smallexample">(define_subst &quot;add_clobber_subst&quot;
  [(set (match_operand:SI 0 &quot;&quot; &quot;&quot;)
        (match_operand:SI 1 &quot;&quot; &quot;&quot;))]
  &quot;&quot;
  [(set (match_dup 0)
        (match_dup 1))
   (clobber (reg:CC FLAGS_REG))])
</pre></div>

<p>This <code>define_subst</code> can be applied to any RTL pattern containing
<code>set</code> of mode SI and generates a copy with clobber when it is
applied.
</p>
<p>Assume there is an RTL template for a <code>max</code> instruction to be used
in <code>define_subst</code> mentioned above:
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;maxsi&quot;
  [(set (match_operand:SI 0 &quot;register_operand&quot; &quot;=r&quot;)
        (max:SI
          (match_operand:SI 1 &quot;register_operand&quot; &quot;r&quot;)
          (match_operand:SI 2 &quot;register_operand&quot; &quot;r&quot;)))]
  &quot;&quot;
  &quot;max\t{%2, %1, %0|%0, %1, %2}&quot;
 [&hellip;])
</pre></div>

<p>To mark the RTL template for <code>define_subst</code> application,
subst-attributes are used.  They should be declared in advance:
</p>
<div class="smallexample">
<pre class="smallexample">(define_subst_attr &quot;add_clobber_name&quot; &quot;add_clobber_subst&quot; &quot;_noclobber&quot; &quot;_clobber&quot;)
</pre></div>

<p>Here &lsquo;<samp>add_clobber_name</samp>&rsquo; is the attribute name,
&lsquo;<samp>add_clobber_subst</samp>&rsquo; is the name of the corresponding
<code>define_subst</code>, the third argument (&lsquo;<samp>_noclobber</samp>&rsquo;) is the
attribute value that would be substituted into the unchanged version of
the source RTL template, and the last argument (&lsquo;<samp>_clobber</samp>&rsquo;) is the
value that would be substituted into the second, transformed,
version of the RTL template.
</p>
<p>Once the subst-attribute has been defined, it should be used in RTL
templates which need to be processed by the <code>define_subst</code>.  So,
the original RTL template should be changed:
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;maxsi&lt;add_clobber_name&gt;&quot;
  [(set (match_operand:SI 0 &quot;register_operand&quot; &quot;=r&quot;)
        (max:SI
          (match_operand:SI 1 &quot;register_operand&quot; &quot;r&quot;)
          (match_operand:SI 2 &quot;register_operand&quot; &quot;r&quot;)))]
  &quot;&quot;
  &quot;max\t{%2, %1, %0|%0, %1, %2}&quot;
 [&hellip;])
</pre></div>

<p>The result of the <code>define_subst</code> usage would look like the following:
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;maxsi_noclobber&quot;
  [(set (match_operand:SI 0 &quot;register_operand&quot; &quot;=r&quot;)
        (max:SI
          (match_operand:SI 1 &quot;register_operand&quot; &quot;r&quot;)
          (match_operand:SI 2 &quot;register_operand&quot; &quot;r&quot;)))]
  &quot;&quot;
  &quot;max\t{%2, %1, %0|%0, %1, %2}&quot;
 [&hellip;])
(define_insn &quot;maxsi_clobber&quot;
  [(set (match_operand:SI 0 &quot;register_operand&quot; &quot;=r&quot;)
        (max:SI
          (match_operand:SI 1 &quot;register_operand&quot; &quot;r&quot;)
          (match_operand:SI 2 &quot;register_operand&quot; &quot;r&quot;)))
   (clobber (reg:CC FLAGS_REG))]
  &quot;&quot;
  &quot;max\t{%2, %1, %0|%0, %1, %2}&quot;
 [&hellip;])
</pre></div>

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